1. Field of the Invention
The present invention relates to a lubricating arrangement for a plastic extruder for the production of cylindrical plastic tubes with a smooth inner wall, said plastic extruder comprising a mandrel, consisting of a form core which is a cylindrical coaxial extension of the extruder screw and rotates with the latter, and a stationary external die means with the lubricating arrangement being provided on the external die means for lubricating its forming surface.
2. Prior Art
From NO-PS No. 158 087 such a lubricating arrangement is known which comprises slots between members of the external die means, said members being movable relative to each other in the axial direction of the die for adjustment of the width of slots. The slot or slots are connected with a source of lubricant, supplying lubricant under pressure. In the embodiment preferred in practice the movable members consist of annular disks. This lubricating arrangement does indeed function as intended, and adjustment of the amount of lubricant is very readily achieved by adjusting the slot width. A disadvantage of this lubricating arrangement, however, is that the slots supplying the die wall with lubricant extend radially into the die wall and normally to the centre line of the external die means in the die wall, whereas the supplied plastic mass describes a helical line due to axial forward squeezing of the mass from the extruder and the influence of the rotating mandrel on the plastic mass. The lubricator slots will, thus, form an acute angle to the helical path of the plastic mass, and this will cause non-uniform lubrication, especially at the inlet to the stationary external die means where shaping of the tube profile begins. This will be especially disadvantageous in connection with production of tubes with a smooth inner surface and with ribs following a helical line on their outside. Such extrusion is very sensitive to non-uniform lubrication, and there will easily be a hazard of uneven extrusion on even a hazard of tearing the tube being moulded. It is an object of the present invention to provide an improved lubricating system, in which the above disadvantage is avoided and by means of which lubricating arrangement uniform lubrication is achieved, especially when tubes with a smooth internal surface and a ribbed external surface are manufactured.